1. Field of the Invention
The invention relates to cell scheduling in Asynchronous Transfer Mode (ATM) networks, and more specifically to weighted fair queuing in ATM networks.
2. Background Art
Ideally, fair cell schedulers should continuously subdivide available bandwidth among competing virtual channels (VCs) according to their weights. While such an hypothetical scheduler has not been built, existing queuing methods try to approximate this model. However, the complexity of these methods, and the resulting processing time required, make them undesirable for high-speed ATM switches, which require a new cell to be selected for transmission every few microseconds. In addition, ATM switches often service tens of thousands of virtual channels, multiplexed onto a single physical link, with connection throughput requirements varying over a wide range, further compounding the requirement for fast scheduling methods.
A simple prior art scheduling method is described in FIG. 1. According to the method of FIG. 1, each VC is allowed to transmit a burst equal to its weight. A variety of VC entries 10 are shown on timeline 12, indicating the order in which they are transmitted. Thus, if VC1 has a weight=3, VC2 has weight=5, VC3 has weight=1 and VC4 has weight=2, VC1 will transmit at times 1, 2 and 3, VC2 will transmit at times 4, 5, 6, 7 and 8, VC3 will transmit at time 9 and VC4 will transmit at times 10 and 11.